Imaging member

ABSTRACT

An imaging member is disclosed which includes a charge generating layer and a charge transport layer. The charge transport layer includes a first charge transport layer adjacent the charge generating layer and at least a second charge transport layer adjacent the first charge transport layer. Each charge transport layer includes a film forming polymer binder and a charge transport material dispersed therein. The first charge transport material has a composition which is the same or different than the second charge transport material and of a lower concentration. It may be selected from an aryl amine, aryl diamine, or combinations thereof. The combination has the effect of reducing charge deficient spots.

BACKGROUND

There is disclosed herein, in various embodiments, an imaging member forreducing charge deficient spots. In the embodiments, the imaging memberhas a charge generating layer and a charge transport layer with multipleconcentrations or regions of particular small molecule charge transportmaterials. The region or layer closest in proximity to the chargegenerating layer comprises a lower concentration of charge transportmaterial than a layer spaced from the charge generating layer. Sucharrangements and compositions reduce charge injection from the chargegenerating layer into the charge transport layer thereby reducing theformation of charge deficient spots.

A typical electrophotographic imaging member is imaged by uniformlydepositing an electrostatic charge on an imaging surface of theelectrophotographic imaging member and then exposing the imaging memberto a pattern of activating electromagnetic radiation, such as light,which selectively dissipates the charge in the illuminated areas of theimaging member while leaving behind an electrostatic latent image in thenon-illuminated areas. This electrostatic latent image may then bedeveloped to form a visible image by depositing finely dividedelectroscopic marking toner particles on the imaging member surface. Theresulting visible toner image can then be transferred to a suitablereceiving member, such as paper.

A number of current electrophotographic imaging members are multilayeredphotoreceptors that, in a negative charging system, comprise a substratesupport, an electrically conductive layer, an optional charge blockinglayer, an optional adhesive layer, a charge generating layer, a chargetransport layer, and optional protective or overcoating layer(s). Themultilayered photoreceptors can take several forms, for example,flexible belts, rigid drums, flexible scrolls, and the like. Flexiblephotoreceptor belts may either be seamed or seamless belts. An anti-curllayer may be employed on the back side of the flexible substratesupport, the side opposite to the electrically active layers, to achievea desired photoreceptor belt flatness.

Although excellent toner images may be obtained with multilayered beltphotoreceptors, a delicate balance in charging image and biaspotentials, and characteristics of toner/developer must be maintained.This places additional constraints on photoreceptor manufacturing, andthus, on the manufacturing yield. Localized microdefect sites, varyingin size of from about 5 to about 200 microns, can sometimes occur inmanufacture, which appear as print defects (microdefects) in the finalimaged copy. In charged area development, where the charged areas areprinted as dark areas, the sites print out as white spots. Thesemicrodefects are called microwhite spots. In discharged area developmentsystems, where the exposed area (discharged area) is printed as darkareas, these sites print out as dark spots on a white background. All ofthese microdefects, which exhibit inordinately large dark decay, arecalled charge deficient spots (CDS). Since the microdefect sites arefixed in the photoreceptor, the spots are registered from one cycle ofbelt revolution to next. Charge deficient spots have been a seriousproblem for a very long time in many organic photoreceptors, such asmulti-layered benzimidazole perylene photoreceptors where the perylenepigment is dispersed in a matrix of a bisphenol Z type polycarbonatefilm forming binder.

Whether these localized microdefect or charge deficient spot sites willshow up as print defects in the final document depends, to some degree,on the development system utilized and, thus, on the machine designselected. For example, some of the variables governing the final printquality include the surface potential of photoreceptor, the imagepotential of the photoreceptor, photoreceptor to development rollerspacing, toner characteristics (such as size, charge, and the like), thebias applied to the development rollers and the like. The imagepotential depends on the light level selected for exposure. The defectsites are discharged, however, by the dark discharge rather than by thelight. The copy quality from generation to generation is maintained in amachine by continuously adjusting some of the parameters with cycling.Thus, defect levels may also change with cycling.

Techniques have been developed for the detection of CDS's. These havelargely involved destructive testing, although some contactless methodshave been developed. Additionally, multilayer imaging members have beendeveloped to block charge injection from the substrate which can giverise to CDS's.

Furthermore, despite the various known photoreceptor designs, thereremains a need in the art for methods of reducing the occurrence ofcharge deficient spots in the first instance and/or to mitigate theireffect in the photoreceptor during use. If the occurrence of chargedeficient spots can be reduced or eliminated, or if their effect in thephotoreceptor during use can be mitigated, then resultant print qualityusing the photoreceptors will increase and photoreceptor productionyield should also increase. Longer photoreceptor useful life ifparticularly desired, for example, because it makes image developmentand machine service more cost effective, and provides increased customersatisfaction.

CROSS REFERENCE TO RELATED APPLICATIONS

The following applications, the disclosures of each being totallyincorporated herein by reference, are mentioned:

U.S. application Ser. No. 10/744,369, filed Dec. 23, 2003, entitled“Imaging Members,” by Satchidanand Mishra, et al. discloses a chargetransport layer in which the concentration of a charge transportmaterial decreases, such as by a decreasing concentration gradient, fromthe lower surface to an upper surface in the charge transport layer.

U.S. application Ser. No. 10/736,864, filed Dec. 16, 2003, entitled“Imaging Members,” by Anthony M. Horgan, et al. discloses a chargetransport layer of an imaging member which includes a plurality ofcharge transport layers coated from solutions of similar or differentcompositions or concentrations, wherein the upper or additionaltransport layer or layers comprise a lower concentration of chargetransport material than the first (bottom) charge transport layer.

U.S. application Ser. No. 10/320,808, filed Dec. 16, 2002, entitled“Imaging Members,” by Anthony M. Horgan et al discloses a dual chargetransport layer in which the top layer comprises a hindered phenoldopant.

INCORPORATION BY REFERENCE

The following patents, the disclosures of which are incorporated intheir entireties by reference, are mentioned:

Electrophotographic imaging members having at least two electricallyoperative layers including a charge generating layer and a transportlayer comprising a diamine are disclosed in U.S. Pat. Nos. 4,265,990;4,233,384; 4,306,008; 4,299,897; and, 4,439,507.

U.S. Pat. No. 5,830,614 relates to a photoreceptor which comprises asupport layer, a charge generating layer, and two charge transportlayers. A first of the charge transport layers consists of chargetransporting polymer comprising a polymer segment in direct linkage to acharge transporting segment and a second transport layer comprises acharge transporting polymer as for the first layer, except that it has alower weight percent of the charge transporting segment than that of thefirst charge transport layer.

U.S. Pat. No. 6,294,300 discloses a photoconductor which includes acharge transport layer coated over a charge generator layer. A holetransport molecule is intentionally added to the charge generator layerpreventing migration of hole transport molecules from the chargetransport layer to the charge generator layer.

U.S. Pat. Nos. 5,703,487 and 6,008,653 disclose methods for detectingCDS's. In the '487 patent, a process for ascertaining the microdefectlevels of an electrophotographic imaging member includes measuringeither the differential increase in charge over and above the capacitivevalue or measuring reduction in voltage below the capacitive value of aknown imaging member and of a virgin imaging member and comparingdifferential increase in charge over and above the capacitive value orthe reduction in voltage below the capacitive value of the known imagingmember and of the virgin imaging member.

U.S. Pat. No. 6,008,653 discloses a method for detecting surfacepotential charge patterns in an electrophotographic imaging member witha floating probe scanner. The scanner includes a capacitive probe, whichis optically coupled to a probe amplifier, and an outer Faraday shieldelectrode connected to a bias voltage amplifier. The probe is maintainedadjacent to and spaced from the imaging surface to form a parallel platecapacitor with a gas between the probe and the imaging surface. Aconstant voltage charge is applied to the imaging surface prior toestablishing relative movement of the probe and the imaging surface.Variations in surface potential are measured with the probe andcompensated for variations in distance between the probe and the imagingsurface. The compensated voltage values are compared to a baselinevoltage value to detect charge patterns in the electrophotographicimaging member

U.S. Pat. Nos. 5,591,554; 5,576,130; and 5,571,649 disclose methods forpreventing charge injection from substrates which give rise to CDS's.These patents disclose an electrophotographic imaging member including asupport substrate having a two layered electrically conductive groundplane layer comprising a layer comprising zirconium over a layercomprising titanium, a hole blocking layer, and an adhesive layer. Theadhesive layer of the '554 patent includes a copolyester film formingresin, and the member further includes an intermediate layer comprisinga carbazole polymer, a charge generation layer comprising a perylene ora phthalocyanine, and a hole transport layer, which is substantiallynon-absorbing in the spectral region at which the charge generationlayer generates and injects photogenerated holes. The adhesive layer ofthe '130 patent comprises a thermoplastic polyurethane film formingresin. The adhesive layer of the '649 patent comprises a polymer blendcomprising a carbazole polymer and a film forming thermoplastic resin incontiguous contact with a hole blocking layer.

BRIEF DESCRIPTION

The present disclosure relates, in various exemplary embodiments, to animaging member which reduces the occurrence of charge deficient spots,and methods of formation and use.

In one aspect, the imaging member includes an optional substrate, asource of charge, and a charge transport layer which receives chargefrom the source. The source of charge generally comprises a chargegenerating layer which comprises photoconductive pigments and a bindermaterial. The charge transport layer includes a film forming polymerbinder and a charge transport material, such as particular smallmolecules of charge transport compounds, or mixtures thereof, dispersedtherein. The charge transport layer includes a first region and a secondregion. The second region is spaced from the source of charge by thefirst region. The first region has a lower concentration of chargetransport material than the second region whereby charge deficient spotsare reduced as compared with an imaging member formed without the firstregion.

In a further aspect, the concentration of charge transport material ofthe first region is from about 30 weight percent to about 90 weightpercent less than the concentration of the charge transport material ofthe second region, including from about 50 weight percent to about 80weight percent less, and about 65 weight percent to about 75 weightpercent less. In another aspect, the charge transport materials of thelayers differ, and in others they can be the same. In a still furtheraspect, the charge transport materials of the layers can comprise of twoor more different materials. In this regard, it has been found that therate of charge injection from the charge generating layer into thecharge transport layer is related to the particular composition of thesmall molecules in contact with pigment particles of the chargegenerating layer, as well as the distance, location, composition and/orconcentration of the small molecules, in the charge transport layer.

In another aspect, a method is disclosed which comprises forming acharge transport layer on a charge generating layer, includingdepositing a first layer on the charge generating layer. The first layerincludes a film forming polymer binder and a charge transport materialdispersed therein. The method further includes depositing at least onesecond layer directly or indirectly on the first layer such that the atleast one second layer is spaced from the charge generating layer by thefirst layer, the at least one second layer comprising a film formingpolymer binder and a charge transport material dispersed therein, aconcentration of charge transport material in the at least one secondlayer, upon drying, being higher than a concentration of chargetransport material in the first layer. An imaging member produced bysuch a method is also included herein.

In a further aspect, a third layer is optionally deposited on the atleast one second layer, the third layer comprising a film formingpolymer binder and optionally a charge transport material dispersedtherein, a concentration of charge transport material in the thirdlayer, upon drying, being lower than a concentration of charge transportmaterial in an adjacent second layer. Again, a corresponding imagingmember produced by this method is further included herein.

In still another aspect, an imaging member is formed which includes acharge generating layer; a first charge transport layer disposed overthe charge generating layer and comprising a charge transport materialdispersed in a film forming binder, the first charge transport materialbeing selected from an aryl monoamine amine, an aryl diamine of theformula:

where R₁₋₁₀ are independently selected from an alkyl containing fromabout 1 to about 10 carbon atoms and combinations thereof; and a secondcharge transport layer disposed over the first charge transport layerand comprising a second charge transport material, the composition ofthe second charge transport material being the same or different fromthat of the first charge transport material and of a greaterconcentration.

In yet another aspect, an imaging member includes a charge generatinglayer; a first charge transport layer disposed over the chargegenerating layer, the first charge transport layer comprising a firstcharge transport material dispersed in a film forming binder; whereinthe first charge transport material is selected from the groupconsisting of an aryl amine tri(4-methylphenyl)amine, andN,N-di(3,4-dimethyl)phenyl, N-(4-biphenyl)amine, an aryl diamineselected fromN,N′-bis-(4-methoxy-2-methylphenyl)-N—N′-diphenyl-biphenyl-4,4′-diamine,andN,N′-bis-(4-methyphenyl)-N,N′-bis(4-ethylphenyl)-[1,1′-(3,3′-dimethyl)biphenyl]-4,4′-diamine,and mixtures thereof; a second charge transport layer disposed over thefirst charge transport layer, the second charge transport layercomprising a second charge transport material dispersed in a filmforming binder, the composition of the second charge transport materialbeing the same or different than the first charge transport material. Ina further aspect, the concentration of the charge transport materials ofthe first charge transport layer is from about 30 weight percent toabout 90 weight percent less than that of the second charge transportlayer.

There is also provided an image forming apparatus for forming images ona recording medium comprising an electrophotographic imaging memberhaving a charge-retentive surface to receive an electrostatic latentimage thereon, wherein the electrophotographic imaging member comprisesthe arrangements and configurations set forth above and described inmore detail below, a development component to apply a developer materialto the charge-retentive surface to develop the electrostatic latentimage to form a developed image on the charge-retentive surface, atransfer component for transferring the developed image from thecharge-retentive surface to another member or a copy substrate, and afusing member to fuse the developed image to the copy substrate.

These and other non-limiting characteristics of the exemplaryembodiments of this disclosure are discussed in more detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The following is a brief description of the drawings, which arepresented for the purpose of illustrating the exemplary embodimentsdisclosed herein and not for the purposes of limiting the same.

FIG. 1 is a schematic cross sectional view of an exemplary imagingmember according to a first embodiment.

DETAILED DESCRIPTION

Aspects of the exemplary embodiments disclosed herein relate to animaging member, to a method of formation of an imaging member, and to amethod of use of such an imaging member. Although the embodimentsdisclosed herein are applicable to electrophotographic imaging membersin flexible belt configuration and rigid drum form, for reason ofsimplicity, the discussions below are focused upon electrophotographicimaging members in flexible belt designs.

In aspects of the exemplary embodiment disclosed herein, there isprovided an imaging member comprising a photogenerating (chargegenerating) layer with a charge transport layer disposed thereon. Thecharge transport layer has a lower surface which is in contiguouscontact with the charge generating layer, and an upper surface.Additionally, the charge transport layer comprises a film forming binderand a charge transport material, such as particular hole transportmolecules, molecularly dispersed or dissolved therein to form a solidsolution. A first layer of the charge transport layer closest inproximity to the charge generating layer has a lower concentration ofcharge transport material than a second layer spaced from the chargetransport layer. Furthermore, the first charge transport layer has acharge transport material that is of the same or different compositionthan that of the second charge transport material. The concentration ofthe charge transport material in the charge transport layer may increasestepwise, or gradually, as for example, by an increasing concentrationgradient, away from the lower surface toward the upper surface.

In another aspect, the concentration of the charge transport materialmay progressively increase from the region closest in proximity to thephotogenerating layer and then may decrease toward the upper region ofthe charge transport layer.

In a further aspect, the charge transport layer comprises multiplecharge transport layers comprising a first or bottom charge transportlayer comprising a first solid solution of a film forming polymer binderand a charge transport material, and thereover and in contact with thefirst layer, a second solid solution charge transport layer or layers,spaced from the photogenerating layer by the first layer. The secondlayer comprises a second solid solution of a film forming binder andcharge transport material that has a composition which is the same ordifferent from, and having higher concentration of charge transportmaterial, than the first layer. Generally, the first and second chargetransport layers will have different charge transport materials.Optionally the charge transport layer may include one or more additionalsolid solution charge transport layers. The second layer and subsequentadditional charge transport layers each can comprise same or differentfilm forming polymer binder and same or different charge transportmaterial.

In a still further aspect, in the additional layers, the content ofcharge transport material is reduced relative to the second chargetransport layer. The concentration of charge transport material may bereduced, for example, in a stepwise, or graduated, concentrationgradient from the second layer toward the top or uppermost layer. Theadditional charge transport layers can comprise from 1 to about 15layers and, more specifically, from 1 to about 5 layers.

It has been found that the charge injection from a source such as thephotogenerating layer, into the charge transport layer is influenced bythe number (concentration) of charge transport molecules in thevicinity. By providing a layer which suppresses the migration rate ofcharge from the charge generating layer into the charge transport layer,CDS spots in images generated by the imaging member can be significantlyreduced. Both types of CDS spots can be reduced-discharge developmentspots, which appear as microblack spots on white backgrounds, andcharger development spots, which appear as microwhite spots on darkbackgrounds, can be suppressed by lowering the concentration of thecharge transport material in the layer adjacent to the charge generationlayer.

The mobility of the injected charge is also suppressed as a result ofthe lower concentration of charge transport material. Accordingly, theprovision of a second layer which provides a higher charge mobility, forexample, by incorporating a higher concentration of charge transportmaterial, spaced from the charge generation layer, facilitates movementof the charge through the charge transport layer overall. Chargemobility can be expressed in terms of average velocity of the chargepassing through a unit area per unit field of the imaging member.

The additional charge transport layers in the charge transport layer mayalso contain a stabilizing antioxidant such as a hindered phenol. Such aphenol is present in the top most layer of the charge transport layer ina reverse concentration gradient to that of the charge transportmaterial. For example, while, as in one embodiment, the concentration ofthe charge transport material increases from the first or bottom layer(or the layer in closest proximity to the photogenerating layer) anddecreases again toward the top layer in the overall charge transportlayer, the concentration of the hindered phenol increases near the topsurface of the charge transport layer and decreases away from it.Furthermore, in order to achieve enhanced wear resistance results, thetop or uppermost layer or region of the charge transport layer mayfurther include particles dispersions of silica, PTFE, and waxpolyethylene for effective lubrication and wear life extension or beprovided with an overcoat.

Advantages associated with the imaging members of the present exemplaryembodiment include for example, a reduction in charge deficient spots(CDS) in images generated with the imaging member. Additional advantagesmay include the avoidance suppression of early onset of charge transportlayer cracking. Such cracking or micro-cracking can be initiated by theinteraction with effluent of chemical compounds, such as exposure tovolatile organic compounds, like solvents, selected for the preparationof the members and corona emissions from machine charging devices. Suchcracking can lead to copy print out defects and also may adverselyaffect functional characteristics of the imaging member.

Processes of imaging, especially xerographic imaging and printing,including digital printing, are also encompassed herein. Morespecifically, the layered photoconductive imaging members disclosedherein can be selected for a number of different known imaging andprinting processes including, for example, electrophotographic imagingprocesses, especially xerographic imaging and printing processes whereincharged latent images are rendered visible with toner compositions of anappropriate charge polarity. Moreover, the imaging members disclosed areuseful in color xerographic applications, particularly high-speed colorcopying and printing processes and which members are in embodimentssensitive in the wavelength region of, for example, from about 500 toabout 900 nanometers, and in particular from about 650 to about 850nanometers, thus diode lasers can be selected as the light source.

An exemplary embodiment of the multilayered electrophotographic imagingmember of flexible belt configuration is illustrated in FIG. 1 which isan exemplary diagram of a cross-section of an imaging member. Theexemplary imaging member includes an optional support substrate 10having an optional conductive surface layer or layers 12, an optionalhole blocking layer 14, an optional adhesive layer 16, a chargegenerating layer 18, a charge transport layer 20 having two or morelayers or sub-layers, optionally comprising at least a first chargetransport layer 22, a second charge transport layer 24, and optionally athird charge transport layer 26. Optionally an imaging member mayinclude one or more overcoat and/or protective layer(s) 28. Other layersof the imaging member may include, for example, an optional ground striplayer 30, applied to one edge of the imaging member to promoteelectrical continuity with the conductive layer 12 through the holeblocking layer 14. An anti-curl back coating layer 32 may be formed onthe backside of the flexible support substrate. The layers 12, 14, 16,18, 22, 24, 26, 28 may be separately and sequentially deposited on thesubstrate 10 as solutions comprising a solvent, with each layer beingdried before deposition of the next. Alternatively or additionally, oneor more of the layers 24, 26, 28 is applied prior to drying of theprevious layer such that partial mixing at the boundaries of adjacentlayers and/or leaching diffusion of one or more materials from one layerinto the adjacent layer(s) can occur.

In the illustrated embodiment, layer 20 has a lower surface 32 formedfrom the first charge transport layer that is in direct contact with theupper surface of the charge generating layer 18, and an upper surface 34that may be the exposed surface of the imaging member if no overcoatlayer 28 is employed or, where an overcoat layer 28 or layer is used,the upper surface 34 is in direct contact with the overcoat layer 28. Itwill be appreciated that upper surface 34 may be formed from the secondcharge transport layer where the charge transport layer includes onlyfirst charge transport layer 22 and second charge transport layer 24. Asshown in FIG. 1, upper surface 34 is formed from the upper surface ofthird charge transport layer 26. The upper surface of the chargetransport layer is formed from the upper surface of the top most chargetransport layer.

The photoreceptor support substrate 10 may be opaque or substantiallytransparent, and may comprise any suitable organic or inorganic materialhaving the requisite mechanical properties. The entire substrate cancomprise the same material as that in the electrically conductivesurface, or the electrically conductive surface can be merely a coatingon the substrate. Any suitable electrically conductive material can beemployed. Typical electrically conductive materials include copper,brass, nickel, zinc, chromium, stainless steel, conductive plastics andrubbers, aluminum, semitransparent aluminum, steel, cadmium, silver,gold, zirconium, niobium, tantalum, vanadium, hafnium, titanium, nickel,chromium, tungsten, molybdenum, paper rendered conductive by theinclusion of a suitable material therein or through conditioning in ahumid atmosphere to ensure the presence of sufficient water content torender the material conductive, indium, tin, metal oxides, including tinoxide and indium tin oxide, and the like.

The substrate 10 can also be formulated entirely of an electricallyconductive material, or it can be an insulating material includinginorganic or organic polymeric materials, such as, MYLAR™, acommercially available biaxially oriented polyethylene terephthalatefrom DuPont, MYLAR™, with a coated conductive titanium surface,otherwise a layer of an organic or inorganic material having asemiconductive surface layer, such as indium tin oxide, aluminum,titanium, and the like, or exclusively be made up of a conductivematerial such as, aluminum, chromium, nickel, brass, other metals andthe like. The thickness of the support substrate depends on numerousfactors, including mechanical performance and economic considerations.

The substrate 10 may be flexible, being seamed or seamless for flexiblephotoreceptor belt fabrication or it can be rigid for use as an imagingmember for plate design applications. The substrate may have a number ofmany different configurations, such as, for example, a plate, a drum, ascroll, an endless flexible belt, and the like. In one embodiment, thesubstrate is in the form of a seamed flexible belt.

The thickness of the substrate 10 depends on numerous factors, includingflexibility, mechanical performance, and economic considerations. Thethickness of the support substrate 10 may range from about 50micrometers to about 3,000 micrometers; and in embodiments of flexiblephotoreceptor belt preparation, the thickness of substrate 10 is fromabout 50 micrometers to about 200 micrometers for optimum flexibilityand to effect minimum induced photoreceptor surface bending stress whena photoreceptor belt is cycled around small diameter rollers in amachine belt support module, for example, 19 millimeter diameterrollers. The surface of the support substrate is cleaned prior tocoating to promote greater adhesion of the deposited coatingcomposition.

An exemplary substrate support 10 is not soluble in any of the solventsused in each coating layer solution, is optically transparent, and isthermally stable up to a high temperature of about 150° C. A typicalsubstrate support 10 used for imaging member fabrication has a thermalcontraction coefficient ranging from about 1×10⁻⁵/° C. to about 3×10⁻⁵/°C. and a Young's Modulus of between about 5×10⁵ psi (3.5×10⁴ Kg/cm²) andabout 7×10^(s) psi (4.9×10⁴ Kg/cm²).

The conductive layer 12 may vary in thickness depending on the opticaltransparency and flexibility desired for the electrophotographic imagingmember. When a photoreceptor flexible belt is desired, the thickness ofthe conductive layer 12 on the support substrate 10, for example, atitanium and/or zirconium conductive layer produced by a sputtereddeposition process, typically ranges from about 20 Angstroms to about750 Angstroms to enable adequate light transmission for proper backerase, and in embodiments from about 100 Angstroms to about 200Angstroms for an optimum combination of electrical conductivity,flexibility, and light transmission. The conductive layer 12 may be anelectrically conductive metal layer which may be formed, for example, onthe substrate by any suitable coating technique, such as a vacuumdepositing or sputtering technique. Typical metals suitable for use asconductive layer 12 include aluminum, zirconium, niobium, tantalum,vanadium, hafnium, titanium, nickel, stainless steel, chromium,tungsten, molybdenum, combinations thereof, and the like. Where theentire substrate is an electrically conductive metal, the outer surfacethereof can perform the function of an electrically conductive layer anda separate electrical conductive layer may be omitted.

A positive charge (hole) blocking layer 14 may then optionally beapplied to the substrate 10 or to the layer 12, where present.Generally, electron blocking layers for positively chargedphotoreceptors allow the photogenerated holes in the charge generatinglayer 18 at the surface of the photoreceptor to migrate toward thecharge (hole) transport layer below and reach the bottom conductivelayer during the electrophotographic imaging processes. Thus, anelectron blocking layer is normally not expected to block holes inpositively charged photoreceptors, such as, photoreceptors coated with acharge generating layer over a charge (hole) transport layer. Anysuitable hole blocking layer capable of forming an effective barrier toholes injected from the adjacent conductive layer 12 into thephotoconductive or photogenerating layer may be utilized. The charge(hole) blocking layer may include polymers, such as, polyvinylbutyral,epoxy resins, polyesters, polysiloxanes, polyamides, polyurethanes,HEMA, hydroxypropyl cellulose, polyphosphazine, and the like, or maycomprise nitrogen containing siloxanes or silanes, nitrogen containingtitanium or zirconium compounds, such as, titanate and zirconate. Holeblocking layers having a thickness in wide range of from about 50Angstroms (0.005 micrometers) to about 10 micrometers depending on thetype of material chosen for use in a photoreceptor design. Typical holeblocking layer materials include, for example, trimethoxysilyl propylenediamine, hydrolyzed trimethoxysilyl propyl ethylene diamine,N-beta-(aminoethyl) gamma-amino-propyl trimethoxy silane, isopropyl4-aminobenzene sulfonyl, di(dodecylbenzene sulfonyl) titanate, isopropyldi(4-aminobenzoyl)isostearoyl titanate, isopropyltri(N-ethylaminoethylamino)titanate, isopropyl trianthranil titanate,isopropyl tri(N,N-dimethylethy[amino)titanate, titanium-4-amino benzenesulfonate oxyacetate, titanium 4-aminobenzoate isostearate oxyacetate,[H₂N(CH₂)₄]CH₃Si(OCH₃)₂, (gammaminobutyl)-methyl diethoxysilane, and[H₂N(CH₂)₃]CH₃₃Si(OCH₃)₂, (gammaminopropyl)-methyl diethoxysilane, andcombinations thereof, as disclosed in U.S. Pat. Nos. 4,338,387;4,286,033; and, 4,291,110, incorporated herein by reference in theirentireties. Other suitable charge blocking layer polymer compositionsare also described in U.S. Pat. No. 5,244,762 which is incorporatedherein by reference in its entirety. These include vinyl hydroxyl esterand vinyl hydroxy amide polymers wherein the hydroxyl groups have beenpartially modified to benzoate and acetate esters which modifiedpolymers are then blended with other unmodified vinyl hydroxy ester andamide unmodified polymers. An example of such a blend is a 30 molepercent benzoate ester of poly(2-hydroxyethyl methacrylate) blended withthe parent polymer poly(2-hydroxyethyl methacrylate). Still othersuitable charge blocking layer polymer compositions are described inU.S. Pat. No. 4,988,597, which is incorporated herein by reference inits entirety. These include polymers containing an alkylacrylamidoglycolate alkyl ether repeat unit. An example of such an alkylacrylamidoglycolate alkyl ether containing polymer is the copolymerpoly(methyl acrylamidoglycolate methyl ether-co-2-hydroxyethylmethacrylate). The disclosures of these U.S. patents are incorporatedherein by reference in their entireties.

The blocking layer 14 is continuous and may have a thickness of lessthan about 10 micrometers because greater thicknesses may lead toundesirably high residual voltage. In aspects of the exemplaryembodiment, a blocking layer of from about 0.005 micrometers to about 2micrometers facilitates charge neutralization after the exposure stepand optimum electrical performance is achieved. The blocking layer maybe applied by any suitable conventional technique, such as, spraying,dip coating, draw bar coating, gravure coating, silk screening, airknife coating, reverse roll coating, vacuum deposition, chemicaltreatment, and the like. For convenience in obtaining thin layers, theblocking layer may be applied in the form of a dilute solution, with thesolvent being removed after deposition of the coating by conventionaltechniques, such as, by vacuum, heating, and the like. Generally, aweight ratio of blocking layer material and solvent of between about0.05:100 to about 5:100 is satisfactory for spray coating.

The optional adhesive layer 16 may be applied to the hole blocking layer14. Any suitable adhesive layer may be utilized. One well known adhesivelayer includes a linear saturated copolyester reaction product of fourdiacids and ethylene glycol. This linear saturated copolyester consistsof alternating monomer units of ethylene glycol and four randomlysequenced diacids in the above indicated ratio and has a weight averagemolecular weight of about 70,000. If desired, the adhesive layer mayinclude a copolyester resin. The adhesive layer is applied directly tothe hole blocking layer. Thus, the adhesive layer in embodiments is indirect contiguous contact with both the underlying hole blocking layerand the overlying charge generating layer to enhance adhesion bonding toprovide linkage. In embodiments, the adhesive layer is continuous.

Any suitable solvent or solvent mixtures may be employed to form acoating solution of the polyester. Typical solvents includetetrahydrofuran, toluene, methylene chloride, cyclohexanone, and thelike, and mixtures thereof. Any other suitable and conventionaltechnique may be used to mix and thereafter apply the adhesive layercoating mixture to the hole blocking layer. Typical applicationtechniques include spraying, dip coating, roll coating, wire wound rodcoating, and the like. Drying of the deposited wet coating may beeffected by any suitable conventional process, such as oven drying,infra red radiation drying, air drying, and the like.

The adhesive layer 16 may have a thickness of from about 0.01micrometers to about 900 micrometers after drying. In embodiments, thedried thickness is from about 200 micrometers and about 900 micrometers,although thicknesses of from about 0.03 micrometers to about 1micrometer are satisfactory for some applications. At thicknesses ofless than about 0.01 micrometers, the adhesion between the chargegenerating layer and the blocking layer is poor and delamination canoccur when the photoreceptor belt is transported over small diametersupports such as rollers and curved skid plates.

The photogenerating (charge generating) layer 18 may thereafter beapplied to the blocking layer 14 or adhesive layer 16, if one isemployed. To create a functional charge transport layer, chargetransport molecules may be added to a polymeric matrix to make itelectrically active, since the polymer material is itself inherentlyincapable of supporting the injection of photogenerated holes andincapable of allowing the transport of these holes through it. Anysuitable charge generating binder layer 18 including aphotogenerating/photoconductive material, which may be in the form ofparticles and dispersed in a film forming binder, such as an inactiveresin, may be utilized. Examples of photogenerating materials include,for example, inorganic photoconductive materials such as amorphousselenium, trigonal selenium, and selenium alloys selected from the groupconsisting of selenium-tellurium, selenium-tellurium-arsenic, seleniumarsenide and mixtures thereof, and organic photoconductive materialsincluding various phthalocyanine pigment such as the X-form of metalfree phthalocyanine, metal phthalocyanines such as vanadylphthalocyanine and copper phthalocyanine, quinacridones, dibromoanthanthrone pigments, benzimidazole perylene, substituted2,4-diamino-triazines, polynuclear aromatic quinones, and the likedispersed in a film forming polymeric binder. Selenium, selenium alloy,benzimidazole perylene, and the like and mixtures thereof may be formedas a continuous, homogeneous photogenerating layer. Benzimidazoleperylene compositions are well known and described, for example, in U.S.Pat. No. 4,587,189, the entire disclosure thereof being incorporatedherein by reference. Multi-photogenerating layer compositions may beutilized where a photoconductive layer enhances or reduces theproperties of the photogenerating layer. Other suitable photogeneratingmaterials known in the art may also be utilized, if desired. Thephotogenerating materials selected should be sensitive to activatingradiation having a wavelength between about 600 450 and about 700 to 850nm during the imagewise radiation exposure step in anelectrophotographic imaging process to form an electrostatic latentimage.

Any suitable inactive resin materials may be employed in thephotogenerating layer 18, including those described, for example, inU.S. Pat. No. 3,121,006, the entire disclosure thereof beingincorporated herein by reference. Typical organic resinous bindersinclude thermoplastic and thermosetting resins such as one or more ofpolycarbonates, polyesters, polyamides, polyurethanes, polystyrenes,polyarylethers, polyarylsulfones, polybutadienes, polysulfones,polyethersulfones, polyethylenes, polypropylenes, polyimides,polymethylpentenes, polyphenylene sulfides, polyvinyl butyral, polyvinylacetate, polysiloxanes, polyacrylates, polyvinyl acetals, polyamides,polyimides, amino resins, phenylene oxide resins, terephthalic acidresins, epoxy resins, phenolic resins, polystyrene and acrylonitrilecopolymers, polyvinylchloride, vinylchloride and vinyl acetatecopolymers, acrylate copolymers, alkyd resins, cellulosic film formers,poly(amideimide), styrene-butadiene copolymers,vinylidenechloride/vinylchloride copolymers, vinylacetate/vinylidenechloride copolymers, styrene-alkyd resins, and the like.

The photogenerating material can be present in the resinous bindercomposition in various amounts. Generally, from about 5 percent byvolume to about 90 percent by volume of the photogenerating material isdispersed in about 10 percent by volume to about 95 percent by volume ofthe resinous binder, and more specifically from about 20 percent byvolume to about 30 percent by volume of the photogenerating material isdispersed in about 70 percent by volume to about 80 percent by volume ofthe resinous binder composition.

The photogenerating layer 18 containing the photogenerating material andthe resinous binder material generally ranges in thickness of from about0.1 micrometer to about 5 micrometer for example, from about 0.3micrometers to about 3 micrometers when dry. The photogenerating layerthickness is generally related to binder content. Higher binder contentcompositions generally employ thicker layers for photogeneration.

The charge transport layer 20 is a multi-layered charge transport layercomprising two or more individual charge transport layers. Chargetransport layer 20 comprises, at the least, a first charge transportlayer 22 disposed about or over the photogenerating layer 18, and asecond charge transport layer 24 disposed about or over the first chargetransport layer. Optionally, the charge transport layer 20 may includeadditional charge transport layers such as, for example, a third chargetransport layer 26 disposed about or over the second charge transportlayer 24. Further, the charge transport layer may include additionalcharge transport layers beyond third charge transport layer 26. Invarious embodiments, the charge transport layer may include from 1 toabout 15 layers beyond the second charge transport layer.

Each charge transport layer includes a charge transport materialdispersed in a polymeric binder. The charge transport material isgenerally any suitable transparent organic polymer or non-polymericmaterial capable of supporting the injection of photogenerated holes orelectrons from the charge generating layer 18 and capable of allowingthe transport of these holes through the respective charge transportlayers to selectively discharge the surface charge on the imaging membersurface.

In one embodiment, the charge transport layer 20 not only serves totransport holes, but also protects the charge generating layer 18 fromabrasion or chemical attack and may therefore extend the service life ofthe imaging member. In one embodiment the charge transport layer is freeor substantially free of photogenerating materials (e.g., layers 22, 24,and optional additional layers, such as layer 26, each contain less than1% of the concentration of photogenerating materials in the chargegenerating layer 18 and in one embodiment, less than 0.01% thereof. Thelayers or sub-layers 22, 24, and any optional layers, such as layer 26,of the overall charge transport layer 20 are normally transparent in awavelength region in which the electrophotographic imaging member is tobe used when exposure is effected therethrough to ensure that most ofthe incident radiation is utilized by the underlying charge generatinglayer 18. Each charge transport layer should exhibit excellent opticaltransparency with negligible light absorption and neither chargegeneration nor discharge if any, when exposed to a wavelength of lightuseful in xerography, e.g., 4000 to 9000 Angstroms. In the case when thephotoreceptor is prepared with the use of a transparent substrate 10 andalso a transparent conductive layer 12, imagewise exposure or erase maybe accomplished through the substrate 10 with all light passing throughthe back side of the substrate. In this case, the materials of thelayers or sub-layers 22, 24, and optional layers, such as layer 26, neednot transmit light in the wavelength region of use if the chargegenerating layer 18 is sandwiched between the substrate and the chargetransport layer 20. The charge transport layer 20 in conjunction withthe charge generating layer 18 is an insulator to the extent that anelectrostatic charge placed on the charge transport layer is notconducted in the absence of illumination. The first or bottom chargetransport layer 22 and the intermediate and top charge transportlayer(s) should trap minimal charges as the case may be passing throughit.

It has been found that charge deficient spots may be reduced byselecting the type and amount of charge transport material used in thefirst charge transport layer relative to the type and amount of chargetransport material in the second charge transport layer and/or anyadditional charge transport layers. In particular, charge deficientspots may be reduced by employing a first charge transport layercomprising a first charge transport material selected from arylmonoamines, and certain aryl diamines, and mixtures thereof, and asecond charge transport layer comprising a charge transport materialhaving a composition which is the same or different from the firstcharge transport material in the first charge transport layer, but inlarger concentrations.

Materials suitable as the first charge transport material include, butare not limited to, monoamines such as aryl monoamines including, butnot limited to bis(4-methylphenyl)-4-biphenylylamine,bis(4-methoxyphenyl)-4-biphenylylamine,bis-3-methylphenyl)-4-biphenylylamine,bis(3-methoxyphenyl)-4-biphenylylamine-N-phenyl-N-(4-biphenylyl)-p-toluidine,N-phenyl-N-(4-biphenylyl)-p-toluidine,N-phenyl-N-(4-biphenylyl)-m-anisidine, bis(3-phenyl)-4-biphenylylamine,N,N,N-tri[3-methylphenyl]amine, N,N,N-tri[4-methylphenyl]amine,N,N-di(3-methylphenyl)-p-toluidine, N,N-di(4-methylphenyl)-m-toluidine,bis-N,N-[(4′-methyl-4-(1,1′-biphenyl)]-aniline,bis-N,N-[(2′-methyl-4(1,1′-biphenyl)]-aniline,bis-N,N-[2′-methyl-4(1,1′-biphenyl)]-p-toluidine,bis-N,N-[(2′-methyl-4(1,1′-biphenyl)]-m-toluidine,N,N-di-(3,4-dimethylphenyl)-4-biphenylamine (DBA),N,N-bis[4-methylphenyl]-N-[3-phenyldecanoate]amine (TTA-decyl),tri-p-tolylamine (TTA), and the like. Exemplary aryl monoamines includetri-p-tolylamine (TTA), andN,N-bis(3,4-dimethylphenyl)-N-(4-biphenyl)amine (Ae-18).

In one exemplary embodiment, the first charge transport layer includestri-p-tolylamine (TTA), which has a formula of

In another exemplary embodiment, the first charge transport layerincludes N,N-bis(3,4-dimethylphenyl)-N-(4-biphenyl) amine (Ae18), whichhas a formula of

The charge transport material for the first charge transport layer mayalso be an aryl diamine of the formula

wherein R₁₋₄ are independently selected from an alkyl containing 1 toabout 10 carbon atoms; a diamine of the formula

wherein R₅₋₁₀ are independently selected from an alkyl containing 1 toabout 10 carbon atoms, or combinations of such diamines. Exemplary aryldiamines include, but are not limited to, such asN,N′-bis(4-methoxy-2-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamineavailable as HCT-305 from Hodogaya, andN,N′-bis(4-methylphenyl)-N,N′-bis(4-ethylphenyl)-[1,1′-(3,3′-dimethyl)biphenyl]-4,4′-diamine(Ab-16).

In one embodiment, the charge transport material in the first chargetransport layer includesN,N′-bis(4-methoxy-2-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine,which has a formula of

and is currently available as HCT-305 from Hodojaya, Japan.

In another exemplary embodiment, the first charge transport layerincludesN,N′-bis(4-methylphenyl)-N,N′-bis(4-ethylphenyl)-[1,1′-(3,3′-dimethyl)biphenyl]-4,4′-diamine,which has a formula of

and is known as Ab-16.

Material suitable as the second charge transport material for use in thesecond charge transport layer include those described in above-mentionedco-pending applications 10/736,864; 10/744,369; and, 10/320,808, nowU.S. Pat. No. 6,433,089 which issued Aug. 23, 2005, incorporated hereinby reference, which may be used singly or in combination for layers 22and 24. Exemplary charge transporting materials include aromaticdiamines, such as aryl diamines. Exemplary diphenyl diamines suited foruse as the charge material, singly or in combination, are represented bythe molecular Formula I below:

wherein each X is independently selected from the group consisting ofalkyl, hydroxy, and halogen. Typically, the halogen is a chloride. WhereX is alkyl, X can comprise from 1 to about 10 carbon atoms, e.g., from 1to 5 carbon atoms, such as methyl, ethyl, propyl, butyl, and the like.Exemplary aromatic diamines of this type includeN,N′-diphenyl-N,N′-bis(alkylphenyl)-1,1′-biphenyl-4,4-diamines, such asm-TBD, which has the formula(N,N′-diphenyl-N,N′-bis[3-methylphenyl]-[1,1-biphenyl]-4,4′-diamine);and N,N′-diphenyl-N,N′-bis(chlorophenyl)-11′-biphenyl-4,4′-diamine; andcombinations thereof.

The respective charge transport materials are generally dispersed in anelectrically inactive polymeric material to form a solid solution andthereby making this material electrically active. The charge transportmaterials may be added to a film forming polymeric material which isotherwise incapable of supporting the injection of photogenerated holesfrom the generation material and incapable of allowing the transport ofthese holes therethrough. This converts the electrically inactivepolymeric material to a material capable of supporting the injection ofphotogenerated holes from the charge generation layer 18 and capable ofallowing the transport of these holes through respective chargetransport layers of the charge transport layer 20 in order to dischargethe surface charge on the charge transport layer. The charge transportmaterial typically comprises small molecules of an organic compound thatcooperate to transport charge between molecules and ultimately to thesurface of the charge transport layer.

Any suitable inactive resin binder soluble in methylene chloride,chlorobenzene, or other suitable solvent may be employed in therespective charge transport layers. Exemplary binders includepolyesters, polyvinyl butyrals, polycarbonates, polystyrene, polyvinylformals, and combinations thereof. The polymer binder used for thecharge transport layers may be, for example, selected from the group ofpolycarbonates, polyester, polyarylate, polyacrylate, polyether,polysulfone, combinations thereof, and the like. Exemplarypolycarbonates include poly(4,4′-isopropylidene diphenyl carbonate),poly(4,4′-diphenyl-11′-cyclohexene carbonate), and combinations thereof.The molecular weight of the binder can be for example, from about 20,000to about 1,500,000. One exemplary binder of this type is a Makrolon™binder, which is available from Bayer AG and comprisespoly(4,4′-isopropylidene diphenyl)carbonate having a weight averagemolecular weight of about 120,000.

Each charge transport layer of charge transport layer 20 may use adifferent film forming polymer binder. Alternatively, an identicalpolymer binder is used throughout the charge transport layer 20 whichtends to provide improved interfacial adhesion bonding between theindividual charge transport layers such as, for example, layers 22, 24,or 26.

The first charge transport layer has a charge transport materialconcentration generally of about 50% by weight or less. In otherembodiments, the concentration of the first charge transport material inthe first charge transport layer is about 35% by weight or less. Inanother embodiment, the concentration of the first charge transportmaterial in the first charge transport layer is about 20% or less. In afurther embodiment, the concentration of the first charge transportmaterial in the charge transport layer is from about 5 to about 50% byweight. And in still another embodiment, the concentration of the firstcharge transport material in the first charge transport layer is fromabout 20 to about 35% by weight. The balance of the charge transportlayer comprises the polymeric binder. All charge transport materialconcentrations are expressed by weight of the dried layer, unlessotherwise indicated.

The concentration of the second charge transport material in the second,charge transport layer is generally greater than about 40% by weightsuch as, for example, from about 40 to about 90% by weight. In oneembodiment, the concentration of the second charge transport material inthe second charge transport layer is from about 40 to about 60% byweight. In another embodiment, the concentration of the second chargetransport material in the second charge transport layer is about 50% byweight.

Charge deficient spots may be reduced by employing the first chargetransport material in a concentration that is less than theconcentration of the second charge transport material. In one aspect,the concentration of charge transport material of the first region isfrom about 30 weight percent to about 90 weight percent less than theconcentration of the charge transport material of the second region,including from about 50 weight percent to about 80 weight percent less,and about 65 weight percent to about 75 weight percent less. In anotheraspect, the types of charge transport materials of the layers differ,and in others they can be the same. In a still further aspect, thecharge transport materials of the layers can comprise of two or moredifferent material. For example, the first charge transport material maybe present in an amount of from about 20 to about 35% by weight of thefirst charge transport layer, and the second charge transport materialmay be present in an amount of 40 to about 60% by weight of the secondcharge transport layer. In another embodiment, the concentration of thefirst charge transport material is about 35% by weight for the firstcharge transport layer, and the concentration of the second chargetransport material is about 50% by weight of the second charge transportlayer.

The overall thickness of the charge transport layer 20 can be from about5 micrometers to about 200 micrometers and is generally from about 10 toabout 40 microns and more specifically from 20 to 35 microns. Thethickness of the first or bottom charge transport sub-layer 22, whendried, can be from about 0.5 to about 15 micrometers, e.g., about 3-7micrometers. The subsequent sub-layers may have a similar thickness or agreater or lesser thickness, depending on the number of sub-layersemployed. In other embodiments, the first layer 22 may from about 2 toabout 15 microns in thickness and the second layer total thickness canbe from about 10 microns to about 35 microns in thickness.

In various embodiments, the thickness of the first layer 22 is less thanthat of the second layer 24. For example, the ratio of the thickness ofthe second layer 24 to that of the first layer 22 can be, for example,at least about 1.2:1 and in one embodiment, at least 1.5:1 and inanother embodiment, at least about 1.8:1. The ratio can be up to about10:1, or higher. As noted above, the higher ratios are particularlysuited to cases where the concentration ratio is high.

As previously described, the charge transport layer 20 may includeadditional charge transport layers disposed about the second chargetransport layer 24. For example, an imaging member may include a thirdcharge transport layer 26 spaced from the charge generating layer 18 bythe layers 22 and 24. Layer 24 is thus sandwiched between layers 22 and26, with layer 26 providing the upper surface 34 of the charge transportlayer 20. Layer 26 may be in contiguous contact with layer 24, or whereseveral layers 24 are employed, with the uppermost layer 24. Aspreviously described, the charge transport layer 20 may includeadditional layers beyond layer 26.

The additional charge transport layers, such as charge transport layer26, also comprise a charge transport material dispersed in a polymericbinder. The charge transport material in the additional charge transportlayers beyond the second charge transport layer may employ the same or adifferent charge transport material as used in the second chargetransport layer. Additionally, the charge transport material used in theadditional layers beyond the second charge transport layer may employthe same or different polymeric binder as used in the second chargetransport layer. The concentration of the charge transport material inthe additional layers beyond the second charge transport layer may beless than, the same, or greater than the concentration of the chargetransport material employed in the second charge transport layer. In oneaspect, the concentration of the charge transport material in chargetransport layers disposed about the second charge transport layerincreases in successive charge transport layers. In another embodiment,the concentration of the charge transport material in the chargetransport layers above the second charge transport layers decreases insuccessive layers.

For example, in one embodiment, an imaging member comprises a chargetransport layer 20 with a first charge transport layer 22, a secondcharge transport layer 24, and a third charge transport layer 26. Thefirst charge transport layer comprises a first charge transport materialin an amount of from about 5 to about 50% by weight, the second chargetransport layer comprises a second charge transport material in anamount of about 50% by weight or greater, and the third charge transportlayer comprises a third charge transport material that may be the sameor different than the second charge transport material in an amount thatis less than the concentration of the charge transport material in thesecond charge transport layer. The concentration of the charge transportmaterial in the third charge transport layer can be, for example, fromabout 1% to about 95% of the concentration of the charge transportmaterial in the second charge transport layer. In one embodiment, forexample, the third layer 26, may comprise at least about 5 weightpercent and may comprise up to about 50 weight percent of chargetransport material, e.g., from about 5 to about 45 wt %.

The charge transport material in the third transport layer may beapproximately the same or somewhat higher or lower than theconcentration of the first charge transport material in the first chargetransport layer. In one embodiment, the concentration of the thirdcharge transport material in the third charge transport layer is greaterthan the concentration of the second charge transport material in thesecond charge transport layer, and thus, the concentration of chargetransport material in the charge transport layer 20 increases withdistance from the charge generating layer 18.

The thickness of additional layers beyond the second charge transportlayer, such as third layer 26, can be less than the thickness of thesecond layer and can be, in some embodiments, from about 2 microns toabout 10 microns.

In an exemplary embodiment, an imaging member includes a chargetransport layer 20 having a first charge transport layer 22 thatcomprises from about 5 to about 50% by weight of a first chargetransport material selected from an aryl amine such as, for example,tri(4-methylphenyl)amine, and N,N-di(3,4 dimethyl)phenyl, N-(4-biphenyl)amine, an aryl diamine such asN,N′-bis-(4-methoxy-2-methyl-phenyl)-N,N′-diphenyl-biphenyl-4,4′-diamine,and N,N′-bis-(4-methylphenyl)-N,N′-bis-(4-ethylphenyl)-[1,1′-(3,3′-dimethyl)biphenyl]-4,4′-diamine,and mixtures thereof, and a second charge transport layer 24 thatcomprises from about 40 to about 60% by weight of m-TBD.

In another exemplary embodiment, the charge transport layer 20 includesa first charge transport layer 22 that comprises about 50% by weight ofan aryl amine, an aryl diamine such as, for example,N,N′-bis-(4-methoxy-2-methyl-phenyl)-N—N′-diphenyl-biphenyl-4,4′-diamine,and N,N′-bis-(4-methyphenyl)-N,N′-bis(4-ethylphenyl)-[1,1′-(3,3′-dimethyl)biphenyl]-4,4′-diamine, and mixturesthereof, and a second charge transport layer 24 that comprises about 50%by weight of m-TBD.

In still another exemplary embodiment, the charge transport layer 20includes a first charge transport layer 22 that comprises about 35% byweight of an aryl amine, an aryl diamine such as, for example,N,N′-bis-(4-methoxy-2-methyl-phenyl)-N—N′-diphenyl-biphenyl-4,4′-diamine,and N,N′-bis-(4-methyphenyl)-N,N′-bis(4-ethylphenyl)-[1,1′-(3,3′-dimethyl)biphenyl]-4,4′-diamine, and mixturesthereof, and a second charge transport layer 24 that comprises about 50%by weight of m-TBD.

In one exemplary embodiment, the charge transport layer 20 includes afirst charge transport layer 22 that comprises from about 10 to about35% by weight of an aryl amine, an aryl diamine such as, for example,N,N′-bis-(4-methoxy-2-methyl-phenyl)-N—N′-diphenyl-biphenyl-4,4′-diamine,and N,N′-bis-(4-methyphenyl)-N,N′-bis(4-ethylphenyl)-[1,1′-(3,3′-dimethyl)biphenyl]-4,4′-diamine, and mixturesthereof, a second charge transport layer 24 that comprises from about 40to about 60% m-TBD, and optionally a third charge transport layer 26which comprises from about 5 to about 50% m-TBD as the charge transportmaterial. In a variation of this embodiment, layer 22 may be about 10microns in thickness layer 24 about 20 microns in thickness and layer 26about 10 microns in thickness. However it is understood that thethickness of the layers 22, 24, 26 can vary and that layers 22 and 24can even be equal in thickness. An exemplary charge transport layerformed according to FIG. 1 may have a first layer 22 comprising about30% of an aryl amine, an aryl diamine such as, for example, fromN,N′-bis-(4-methoxy-2-methyl-phenyl)-N—N′-diphenyl-biphenyl-4,4′-diamine,and N,N′-bis-(4-methyphenyl)-N,N′-bis(4-ethylphenyl)-[1,1′-(3,3′-dimethyl)biphenyl]-4,4′-diamine, and mixturesthereof as the charge transport material, a second charge transportlayer 24 having a greater thickness than the first layer 22, andcomprising about 50% m-TBD as the charge transport material, and a thirdcharge transport layer 26 comprising less than 50% m-TBD, e.g., about40% or less.

In another exemplary embodiment, layer 22 comprises from about 5 toabout 10% by weight m-TBD and layer 24 comprises from about 20 to about60% m-TBD. In this embodiment, layer 22 may be about 8 microns inthickness and layer 24 about 22 microns in thickness.

Another exemplary charge transport layer formed according to FIG. 1 mayhave a first charge transport layer 22 comprising about 20% by weight ofan aryl amine, an aryl diamine such as, for example, fromN,N′-bis-(4-methoxy-2-methyl-phenyl)-N—N′-diphenyl-biphenyl-4,4′-diamine,and N,N′-bis-(4-methyphenyl)-N,N′-bis(4-ethylphenyl)-[1,1′-(3,3′-dimethyl)biphenyl]-4,4′-diamine, and mixturesthereof as the charge transport material, a second charge transportlayer 24 of greater thickness than the first layer 22 and comprisingabout 55% by weight m-TBD as the charge transport material, and a thirdcharge transport layer 26 having a lower thickness than the second layerand comprising about 30% by weight m-TBD as the charge transportmaterial.

If desired, the composition of the top charge transport layer, such ascharge transport layer 26 in each of the photoreceptors described in theabove embodiments may also include, for example, additions ofantioxidants, leveling agents, surfactants, wear resistant fillers suchas dispersion of polytetrafluoroethylene (PTFE) particles and silicaparticles, light shock resisting or reducing agents, and the like, toimpart further photo-electrical, mechanical, and copy print-out qualityenhancement outcomes, particularly if no overcoat layer 28 is used.

CDS's are suppressed by the layer 22. It may be desirable to employ athird charge transport layer 26 having the lower concentration of thecharge transport material near the exposed surface to reduce problemsarising from corona effluents and solvents in the surroundingatmosphere, such as cracking and lateral charge migration (LCM). Chargetransport materials, such as m-TBD tend to be oxidized by theseeffluents. Thus, a lower concentration in the upper layer 40 mitigatesthese effects,

Additional aspects relate to the inclusion in the charge transport layer20 of variable amounts of an antioxidant, such as a hindered phenol.Exemplary hindered phenols includeoctadecyl-3,5-di-tert-butyl-4-hydroxyhydrociannamate, available asIrganox I-1010 from Ciba Specialty Chemicals. The hindered phenol may bepresent at about 10 weight percent based on the concentration of thecharge transport material. The hindered phenol concentration may betailored to produce a continuum of varying concentration of theantioxidant in reversal to that of the charge transport material forimproved electrical stability and minimization of LCM impact.

Additional aspects relate to inclusion in the upper layer of the chargetransport layer or to an overcoat layer 28 of nano particles as adispersion, such as silica, metal oxides, Acumist™ (waxy polyethyleneparticles), PTFE, and the like. The nanoparticles may be used to enhancethe lubricity and wear resistance of the charge transport layer 20. Theparticle dispersion concentrated in the top vicinity of the upper regionof charge transport layer 20 can be up to about 10 weight percent of theweight of the top region or one tenth thickness of the charge transportlayer 20 to provide optimum wear resistance without causing adeleterious impact on the electrical properties of the fabricatedimaging member.

The charge transport layer 20 is an insulator to the extent that theelectrostatic charge placed on the charge transport layer is notconducted in the absence of illumination at a rate sufficient to preventformation and retention of an electrostatic latent image thereon. Ingeneral, the ratio of the thickness of the charge transport layer 20 tothe charge generator layer 18 is maintained from about 2:1 to about200:1 and in some instances as great as about 400:1.

Other layers such as conventional ground strip layer 30 including, forexample, conductive particles dispersed in a film forming binder may beapplied to one edge of the imaging member to promote electricalcontinuity with the conductive layer 12 through the hole blocking layer14, and adhesive layer 16. Ground strip layer 30 may include anysuitable film forming polymer binder and electrically conductiveparticles. Typical ground strip materials include those enumerated inU.S. Pat. No. 4,664,995, the entire disclosure of which is incorporatedby reference herein. The ground strip layer 30 may have a thickness fromabout 7 micrometers to about 42 micrometers, for example, from about 14micrometers to about 23 micrometers. Optionally, an overcoat layer 28,if desired, may also be utilized to provide imaging member surfaceprotection as well as improve resistance to abrasion and scratching.

Where an overcoat layer 28 is employed, it may comprise a similar resinused for the charge transport layer or a different resin and be fromabout 1 to about 2 microns in thickness.

Since the charge transport layer 20 can have a substantial thermalcontraction mismatch compared to that of the substrate support 10, theprepared flexible electrophotographic imaging member may exhibitspontaneous upward curling due to the result of larger dimensionalcontraction in the charge transport layer 20 than the substrate support10, as the imaging member cools down to room ambient temperature afterthe heating/drying processes of the applied wet charge transport layercoating. An anti-curl back coating 32 can be applied to the back side ofthe substrate support 10 (which is the side opposite the side bearingthe electrically active coating layers) in order to render flatness.

The anti-curl back coating 32 may include any suitable organic orinorganic film forming polymers that are electrically insulating orslightly semi-conductive. The anti-curl back coating 32 used has athermal contraction coefficient value substantially greater than that ofthe substrate support 10 used in the imaging member over a temperaturerange employed during imaging member fabrication layer coating anddrying processes (typically between about 20° C. and about 130° C.). Toyield the designed imaging member flatness outcome, the appliedanti-curl back coating has a thermal contraction coefficient of at leastabout 1.5 times greater than that of the substrate support to beconsidered satisfactory; that is a value of at least approximately1×10⁻⁵/° C. greater than the substrate support, which typically has asubstrate support thermal contraction coefficient of about 2×10⁻⁵/° C.However, an anti-curl back coating with a thermal contractioncoefficient at least about 2 times greater, equivalent to about 2×10⁵/°C. greater than that of the substrate support is appropriate to yield aneffective anti-curling result. The applied anti-curl back coating 32 canbe a film forming thermoplastic polymer, being optically transparent,with a Young's Modulus of at least about 2×10⁵ psi (1.4×10⁴ Kg/cm²),bonded to the substrate support to give at least about 15 gms/cm of 180°peel strength. The anti-curl back coating 32 may be from about 7 toabout 20 weight percent based on the total weight of the imaging member,which may correspond to from about 7 to about 20 micrometers in drycoating thickness. The selected anti-curl back coating is readilyapplied by dissolving a suitable film forming polymer in any convenientorganic solvent.

Exemplary film forming thermoplastic polymers suitable for use in theanti-curl back coating include polycarbonates, polystyrenes, polyesters,polyamides, polyurethanes, polyarylethers, polyarylsulfones,polyarylate, polybutadienes, polysulfones, polyethersulfones,polyethylenes, polypropylenes, polyimides, polymethylpentenes,polyphenylene sulfides, polyvinyl acetate, polysiloxanes, polyacrylates,polyvinyl acetals, polyamides, polyimides, amino resins, phenylene oxideresins, terephthalic acid resins, phenoxy resins, epoxy resins, phenolicresins, polystyrene and acrylonitrile copolymers, polyvinylchloride,vinylchloride and vinyl acetate copolymers, acrylate copolymers, alkydresins, cellulosic film formers, poly(amideimide), styrene-butadienecopolymers, vinylidenechloride-vinylchloride copolymers,vinylacetate-vinylidenechloride copolymers, styrene-alkyd resins,combinations thereof; and the like. These polymers may be block, randomor alternating copolymers. Molecular weights can vary from about 20,000to about 150,000. Suitable polycarbonates include bisphenol Apolycarbonate materials, such as poly(4,4′-isopropylidene-diphenylenecarbonate) having a molecular weight of from about 35,000 to about40,000, available as Lexan 145™ from General Electric Company andpoly(4,4′-isopropylidene-diphenylene carbonate) having a molecularweight of from about 40,000 to about 45,000, available as Lexan 141™also from the General Electric Company. A bisphenol A polycarbonateresin having a molecular weight of from about 50,000 to about 120,000,is available as Makrolon™ from Farbenfabricken Bayer A.G. A lowermolecular weight bisphenol A polycarbonate resin having a molecularweight of from about 20,000 to about 50,000 is available as Merlon™ fromMobay Chemical Company. Another suitable polycarbonate ispoly(4,4-diphenyl-1,1′-cyclohexene carbonate), which is a film formingthermoplastic polymer comprising a structurally modified from bisphenolA polycarbonate which is commercially available from MitsubishiChemicals. All of these polycarbonates have a Tg of between about 145°C. and about 165° C. and with a thermal contraction coefficient rangingfrom about 6.0×10⁻⁵/° C. to about 7.0×10⁻⁵/° C.

Furthermore, suitable film forming thermoplastic polymers for theanti-curl back coating 32, if desired, may include the same binderpolymers used in the charge transport layer 20. The anti-curl backcoating formulation may include a small quantity of a saturatedcopolyester adhesion promoter to enhance its adhesion bond strength tothe substrate support. Typical copolyester adhesion promoters are Vitel™polyesters from Goodyear Rubber and Tire Company, Mor-Ester™ polyestersfrom Morton Chemicals, Eastar PETG™ polyesters from Eastman Chemicals,and the like. To impart optimum wear resistance as well as maintainingthe coating layer optical clarity, the anti-curl layer may furtherincorporate in its material matrix, about 5 to about 30 weight percentfiller dispersion of silica particles, Teflon™ particles, PVF₂particles, stearate particles, aluminum oxide particles, titaniumdioxide particles or a particle blend dispersion of Teflon and any ofthese inorganic particles. Suitable particles used for dispersion in theanti-curl back coating include particles having a size of between about0.05 and about 0.22 micrometers, and more specifically between about0.18 and about 0.20 micrometers.

In one embodiment, the anti-curl back coating 32 is opticallytransparent. The term optically transparent is defined herein as thecapability of the anti-curl back coating to transmit at least about 98percent of an incident light energy through the coating. The anti-curlback coating of this embodiment includes a film forming thermoplasticpolymer and may have a glass transition temperature (Tg) value of atleast about 75° C., a thermal contraction coefficient value of at leastabout 1.5 times greater than the thermal contraction coefficient valueof the substrate support, a Young's Modulus of at least about 2×10⁵p.s.i., and adheres well over the supporting substrate to give a 180°peel strength value of at least about 15 g/cm.

The multilayered, flexible electrophotographic imaging member web stockshaving the charge transport layer fabricated in accordance with theembodiments described herein may be cut into rectangular sheets. Eachcut sheet is then brought overlapped at ends thereof and joined by anysuitable means, such as ultrasonic welding, gluing, taping, stapling, orpressure and heat fusing to form a continuous imaging member seamedbelt, sleeve, or cylinder.

The prepared flexible imaging belt may thereafter be employed in anysuitable and conventional electrophotographic imaging process whichutilizes uniform charging prior to imagewise exposure to activatingelectromagnetic radiation, When the imaging surface of anelectrophotographic member is uniformly charged with an electrostaticcharge and imagewise exposed to activating electromagnetic radiation,conventional positive or reversal development techniques may be employedto form a marking material image on the imaging surface of theelectrophotographic imaging member Thus, by applying a suitableelectrical bias and selecting toner having the appropriate polarity ofelectrical charge, a toner image is formed in the charged areas ordischarged areas on the imaging surface of the electrophotographicimaging member. For example, for positive development, charged tonerparticles are attracted to the oppositely charged electrostatic areas ofthe imaging surface and for reversal development, charged tonerparticles are attracted to the discharged areas of the imaging surface.

The development will further be illustrated in the followingnon-limiting examples, it being understood that these examples areintended to be illustrative only and that the disclosure is not intendedto be limited to the materials, conditions, process parameters and thelike recited herein. All proportions are by weight unless otherwiseindicated.

EXAMPLES

In the following Examples, imaging members with two charge transportlayers were prepared to demonstrate the reduction in CDS by employing alayer of lower concentration of charge transport molecules adjacent thecharge generation layer. It will be appreciated that these imagingmembers can be prepared with three transport layers or with gradientlayers to provide a peak concentration intermediate the surfacecontacting the charge generation layer and the upper surface of thecharge transport layer.

Examples 1-6

Imaging members were separately prepared by providing a 0.02 micrometerthick titanium layer coated on a biaxially oriented polyethylenenaphthalate substrate (KALEDEX™ 2000) having a thickness of 3.5 mils(0.09 millimeters). Applied thereon with a gravure applicator, was asolution containing 50 grams 3-amino-propyltriethoxysilane, 41.2 gramswater, 15 grams acetic acid, 684.3 grams of 200 proof denatured alcoholand 200 grams heptane. This layer was then dried for about 2 minutes at120° C. in the forced air drier of the coater. The resulting blockinglayer had a dry thickness of 500 Angstroms.

An adhesive layer was then prepared by applying a wet coating over theblocking layer, using a gravure applicator, containing 0.2 percent byweight based on the total weight of the solution of polyarylate adhesive(Ardel™ D100 available from Toyota Hsutsu Inc.) in a 60:30:10 volumeratio mixture of tetrahydrofuran/monochlorobenzene/methylene chloride.The adhesive layer was then dried for about 2 minutes at 120° C. in theforced air dryer of the coater. The resulting adhesive layer had a drythickness of 200 Angstroms.

A photogenerating layer dispersion was prepared by introducing 0.45grams of Lupilon™ 200 (PC-Z 200) available from Mitsubishi Gas ChemicalCorp and 50 ml of tetrahydrofuran (THF) into a 100 gm glass bottle. Tothis solution was added 2.4 grams of hydroxygallium phthalocyanine and300 grams of ⅛ inch (3.2 millimeter) diameters stainless steel shot.This mixture was then placed on a ball mill for 8 hours. Subsequently,2.25 grams of PC-Z 200 was dissolved in 46.1 gm of tetrahydrofuran, andadded to this OHGaPc slurry. This slurry was then placed on a shaker for10 minutes. The resulting slurry was, thereafter, applied to theadhesive interface with a Bird applicator to form a charge generationlayer having a wet thickness of 0.25 mil (about 6 microns). However, astrip about 10 mm wide along one edge of the substrate web wearing theblocking layer and the adhesive layer, was deliberately left uncoatedwithout any photogenerating layer material, to facilitate adequateelectrical contact by the ground strip layer that was to be appliedlater. The charge generation layer was dried at 120° C. for 1 minute ina forced air oven to form a dry charge generation layer having athickness of 0.4 micrometers.

This photogenerator layer was overcoated with a first charge transportlayer. The first charge transport layer was prepared by introducing intoan amber glass bottle a first charge transport material and a polymericbinder. The charge transport material, binder and weight ratios ofcharge transport material are set forth in Table 1.

TABLE 1 Charge Transport Materials for the First Charge Transport LayerFirst Charge Charge Transport Example Transport Polymer Material toBinder Control 1 m-TBD Makrolon ™ 50:50 Control 2 m-TBD Makrolon ™ 35:651 ACT-305 Makrolon ™ 50:50 2 ACT-305 Makrolon ™ 35:65 3 Ae-18 Makrolon ™50:50 4 Ae-18 Makrolon ™ 35:65 5 TTA Makrolon ™ 50:50 6 TTA Makrolon ™35:65The resulting mixture was dissolved in methylene chloride to form asolution containing 15 percent by weight solids. This solution wasapplied on the photogenerator layer using a Bird applicator to form acoating which upon drying had a thickness of 14.5 microns. During thiscoating process the humidity was equal to or less than 15 percent.

This first charge transport layer was overcoated with a second chargetransport layer. The second charge transport layer was prepared byintroducing into an amber glass bottle in a weight ratio of50:50N,N′-diphenyl-N,N′-bis(3-methylphenyl)-biphenyl-4,4-diamine andMakrolon™ 5705. The resulting mixture was dissolved in methylenechloride to form a solution containing 15 percent by weight solids. Thissolution was applied on the photogenerator layer using a Bird applicatorto form a coating which upon drying had a thickness of 14.5 microns.During this coating process the humidity was equal to or less than 15percent.

Example 7 Electrical Scanner

The flexible photoreceptor sheets prepared as described in Examples 1-6were tested for their xerographic sensitivity and cyclic stability in ascanner. In the scanner, each photoreceptor sheet to be evaluated wasmounted on a cylindrical aluminum drum substrate, which was rotated on ashaft. The devices were charged by a corotron mounted along theperiphery of the drum. The surface potential was measured as a functionof time by capacitatively coupled voltage probes placed at differentlocations around the shaft. The probes were calibrated by applying knownpotentials to the drum substrate. Each photoreceptor sheet on the drumwas exposed to a light source located at a position near the drumdownstream from the corotron. As the drum was rotated, the initial(pre-exposure) charging potential (Vddp) was measured by a first voltageprobe. Further rotation lead to an exposure station, where thephotoreceptor device was exposed to monochromatic radiation of a knownintensity of 3.5 ergs/cm² to obtain Vbg. The devices were erased by alight source located at a position upstream of charging to obtain Vr.The measurements illustrated in Table 2 below include the charging ofeach photoconductor device in a constant current or voltage mode. Thedevices were charged to a negative polarity corona. The surfacepotential after exposure (Vbg) was measured by a second voltage probe.In the design, the exposure could be turned off in certain cycles. Thevoltage measured at the second probe is then Vddp. The voltage generallyis higher at the changing station. The difference between the chargedvoltage at the charging station and the Vddp is dark decay. The deviceswere finally exposed to an erase lamp of appropriate intensity and anyresidual potential (Vr) was measured by a third voltage probe. After10,000 charge-erase cycles, the Vbg was remeasured and the differencebetween Vbg for the first cycle and Vbg for cycle 10,000 (ΔVbg 10 K) wascomputed.

TABLE 2 Vbg (initial) Vbg (10k) Constant 3.5 erg/cm²; 3.5 erg/cm²;Vresidual QV Current Example Vddp = 500 Vddp = 500 (300 erg/cm²) DarkDecay Slope Vo Control 1 54 99 17 −206 51 1113 Control 2 76 128 37 −14852 1154 1 53 128 17 −139 50 1086 2 72 85 48 −145 51 1120 3 77 135 33−139 53 1159 4 92 153 60 −140 49 1119 5 87 154 39 −135 54 1191 6 95 16559 −136 50 1126

The sheets thus formed were tested with a floating probe scanner (FPSscanner) for CDS in a manner similar to that described in U.S. Pat. No.6,008,653 and U.S. Pat. No. 6,119,536, incorporated herein by reference.The 23 cm wide and 28 cm long sheets of all the samples were cut andmounted on a drum of the FPS scanner one at a time. The drum was rotatedcontinuously and underwent a sequence of charging under a scorotron to700 volts. Then measurements of micro defects were made. These consistedof high resolution voltage measurements of 50 to 100 micron resolutionby an aerodynamically floating probe which was capacitively coupled tothe photoreceptor charged surface. The probe was maintained at aconstant of 50 microns during the entire scan of the sample surface.After this, the photoreceptor was discharged by an erase lamp before thenext cycle started. In each cycle the drum was moved translationally insmall steps of 25 to 50 microns. The floating probe scanner then countedthe CDS's over an area of about 100 to 150 cm² and provided an averagevalue/cm². Table 2 shows the electrical properties. Table 3 shows theresults for the CDS tests.

TABLE 3 First Charge Concentration of Transport First Charge ExampleMaterial Transport Material CDS/cm² Control 1 m-TBD 50 12.7 Control 2m-TBD 35 8.2 1 ACT-305 50 27.9 2 ACT-305 35 10.9 3 Ae-18 50 7.3 4 Ae-1835 3.0 5 TTA 50 9.3 6 TTA 35 3.8

As the CDS results show, employing a first charge transport layer with acharge transport material different from the charge transport materialin the second charge transport layer, and in particular from the classof materials described herein, is beneficial to reducing CDS. Table 3shows that CDS is reduced relative to the control even when the firstcharge transport material is employed in the same concentration as thesecond charge transport material in the second charge transport layer.CDS is reduced further, however, by employing the first charge transportlayer in an amount that is lower than the concentration of the secondcharge transport material in the second charge transport layer.

It will be appreciated that various of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be desirablycombined into many other different systems or applications. Also thatvarious presently unforeseen or unanticipated alternatives,modifications, variations or improvements therein may be subsequentlymade by those skilled in the art which are also intended to beencompassed by the following claims.

1. An imaging member comprising: a charge generating layer; a firstcharge transport layer disposed over the charge generating layer andcomprising a first charge transport material, the first charge transportmaterial comprising an aryl diamine of the formula

where R₁-R₄ are independently selected from an alkyl containing fromabout 1 to about 10 carbon atoms and mixtures thereof; and a secondcharge transport layer disposed over the first charge transport layerand comprising a second charge transport material, the second chargetransport material comprising a composition which is the same ordifferent from that of the first charge transport material and of agreater concentration.
 2. The imaging member according to claim 1,wherein the first charge transport material comprises a mixture ofcharge transport materials.
 3. The imaging member according to claim 1,wherein the concentration of the first charge transport material in thefirst charge transport layer is less than the concentration of thesecond charge transport material in the second charge transportmaterial.
 4. The imaging member according to claim 1, wherein the firstcharge transport layer comprises the first charge transport material inan amount of less than about 35% by weight.
 5. The imaging memberaccording to claim 4, which the second charge transport layer comprisesthe second charge transport material in an amount of about 40% by weightor greater.
 6. The imaging member according to claim 1, wherein thefirst charge transport layer comprises the first charge transportmaterial in an amount of about 20% by weight or less.
 7. The imagingmember according to claim 6, further comprising a third charge transportlayer disposed about the second charge transport layer, wherein thethird charge transport layer comprises a third charge transportmaterial.
 8. The imaging member according to claim 7, wherein theconcentration of the third charge transport material is greater than theconcentration of the second charge transport material which is greaterthan the concentration of the first charge transport material.
 9. Theimaging member according to claim 7, wherein the third charge transportmaterial is present in a concentration that is less than theconcentration of the second charge transport material.
 10. The imagingmember according to claim 9, wherein the first charge transport materialis present in a concentration that is less than the concentration of thesecond transport material.
 11. The imaging member according to claim 1,wherein the first charge transport material further comprises an arylamine selected from the group consisting of tri-p-tolylamine;N,N-bis(3,4-dimethylphenyl)-N-(4-biphenyl)amine; and mixtures thereof.12. The imaging member according to claim 1, wherein the first chargetransport material comprises an aryl diamine of the formula


13. A xerographic printing system comprising the imaging member ofclaim
 1. 14. An imaging member comprising: an optional substrate; acharge generating layer; a first charge transport layer disposed overthe charge generating layer, the first charge transport layer comprisinga first charge transport material dispersed in a film forming binder; asecond charge transport layer disposed over the first charge transportlayer, the second charge transport layer comprising a second chargetransport material dispersed in a film forming binder, the second chargetransport material having a composition which is the same or differentfrom that of the first charge transport material; and optionally a thirdcharge transport layer disposed about the second charge transport layer,the third charge transport material having a composition which is thesame or different from that of the second transport material; wherein atleast the first charge transport material comprisesN,N′-bis(4-methoxy-2-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine.15. The imaging member according to claim 14, wherein the first chargetransport material is present in the first charge transport layer in anamount of from about 5 to about 50% by weight of the first chargetransport layer, and the second charge transport material is present inthe second charge transport layer in an amount of from about 40% toabout 90% by weight.
 16. The imaging member according to claim 15,wherein the first charge transport material is present in aconcentration less than the concentration of the second charge transportmaterial.
 17. The imaging member according to claim 15, wherein thefirst charge transport material is present in the first charge transportmaterial in an amount of about 35% by weight, and the second chargetransport material is present in the second charge transport layer in anamount of from about 50% by weight.
 18. The imaging member according toclaim 14, wherein the second charge transport material comprises m-TBD.19. An imaging member comprising: a charge generating layer; a firstcharge transport layer disposed over the charge generating layer andcomprising a first charge transport material; and a second chargetransport layer disposed over the first charge transport layer andcomprising a second charge transport material; wherein the concentrationof the second charge transport material in the second charge transportlayer is greater than the concentration of the first charge transportmaterial in the first charge transport layer; and either the firstcharge transport material or the second charge transport materialcomprises an aryl diamine of the formula

where R₁-R₄ are independently selected from alkyl containing from about1 to about 10 carbon atoms and mixtures thereof.